The natural gamma spectroscopy probe analyses the energy spectrum of gamma radiation from naturally occurring or man-made isotopes in the formation surrounding a borehole. The probe corrects for temperature drift is real-time by matching the acquired spectrum to base spectra of the main natural emitters, potassium, uranium and thorium determined during the tool master calibration. Available outputs are full-spectrum (static mode only) and continuous log measurements of elemental concentrations. Borehole corrections are available for casing thickness, borehole diameter, formation density and mud/fluid radioactivity for both centralized and side-walled tool positions.
Principle of Measurement:
Gamma photons produced by the decay of naturally occurring potassium, uranium, thorium and/or unstable man-made isotopes in the formation are detected by a large-volume gamma scintillation counter and converted to electrical pulses. The amplitude of the pulses depends on the photon energy. An analyzer within the probe separates the pulses into channels according to their amplitudes. Count-rates from groups of channels are converted in real-time by the surface software to concentrations of the originating elements using predetermined algorithms.